The present invention relates to a method and apparatus for separating a sample such as a bonded substrate stack, a transfer film transfer method, a substrate manufacturing method, and a semiconductor device and a method of manufacturing the same.
A substrate (SOI substrate) having an SOI (Silicon On Insulator) structure is known as a substrate having a single-crystal Si layer on an insulating layer. A device using this SOI substrate has many advantages that cannot be achieved by ordinary Si substrates. Examples of the advantages are as follows.
(1) The integration degree can be increased because dielectric isolation is easy.
(2) The radiation resistance can be increased.
(3) The operating speed of the device can be increased because the stray capacitance is small.
(4) No well step is necessary.
(5) Latch-up can be prevented.
(6) A complete depletion type field effect transistor can be formed by thin film formation.
Since an SOI structure has the above various advantages, researches have been made on its formation method for several decades.
As a method, an SOI structure is formed by bonding a single-crystal Si substrate to another thermally oxidized single-crystal Si substrate by annealing or an adhesive. In this method, an active layer for forming a device must be uniformly thin. More specifically, a single-crystal Si substrate having a thickness of several hundred micron must be thinned down to the micron order or less.
To thin the substrate, polishing or selective etching can be used.
A single-crystal Si substrate can hardly be uniformly thinned by polishing. Especially, in thinning to the submicron order, the variation range is several ten %. As the wafer size becomes large, this difficulty becomes more pronounced.
Selective etching is effective to uniformly thin the substrate. However, the selectivity ratio is as low as about 102, the surface planarity after etching is poor, and the crystallinity of the SOI layer is unsatisfactory.
The present applicant has disclosed a new SOI technique in Japanese Patent Laid-Open No. 5-021338. In this technique, a first substrate obtained by forming a porous layer on a single-crystal Si substrate and a non-porous single-crystal layer on its surface is bonded to a second substrate via an insulating layer. After this, the bonded substrate stack is separated into two substrates at the porous layer, thereby transferring the non-porous single-crystal layer to the second substrate. This technique is advantageous because the film thickness uniformity of the SOI layer is good, the crystal defect density in the SOI layer can be decreased, the surface planarity of the SOI layer is good, no expensive manufacturing apparatus with special specifications is required, and SOI substrates having about several hundred-xc3x85 to 10-xcexcm thick SOI films can be manufactured by a single manufacturing apparatus.
The present applicant has also disclosed, in Japanese Patent Laid-Open No. 7-302889, a technique of bonding first and second substrates, separating the first substrate from the second substrate without breaking the first substrate, smoothing the surface of the separated first substrate, forming a porous layer again, and reusing the substrate. Since the first substrate is not wasted, this technique is advantageous in largely reducing the manufacturing cost and simplifying the manufacturing process.
To separate the bonded substrate stack into two substrates without breaking the first and second substrates, the following methods are available: the two substrates are pulled in opposite directions while applying a force in a direction perpendicular to the bonding interface; a shearing force is applied parallel to the bonding interface (for example, the two substrates are moved in opposite directions in a plane parallel to the bonding interface, or the two substrates are rotated in opposite directions while applying a force in the circumferential direction); pressure is applied in a direction perpendicular to the bonding interface; a wave energy such as an ultrasonic wave is applied to the separation region; a peeling member (e.g., a sharp blade such as a knife) is inserted into the separation region parallel to the bonding interface from the side surface side of the bonded substrate stack; the expansion energy of a substance filling the pores of the porous layer functioning as the separation region is used; the porous layer functioning as the separation region is thermally oxidized from the side surface of the bonded substrate stack to expand the volume of the porous layer and separate the substrates; and the porous layer functioning as the separation region is selectively etched from the side surface of the bonded substrate stack to separate the substrates.
Porous Si was found in 1956 by Uhlir et al. who were studying electropolishing of semiconductors (xe2x80x9cElectrolytic Shaping of Germanium and Siliconxe2x80x9d, A. Uhlir, Bell System Technical Journal Vol.35, pp. 333-347, March, 1956).
Porous Si can be formed by anodizing an Si substrate in an HF solution.
Unagami et al. studied the dissolution reaction of Si upon anodizing and reported that holes were necessary for anodizing reaction of Si in an HF solution, and the reaction was as follows (xe2x80x9cFormation Mechanism of Porous Silicon Layer by anodization in HF Solutionxe2x80x9d T. Unagami, Journal of the Electrochemical Society, Vol. 127, pp. 476-483, 1980T. Unagami).
Si+2HF+(2xe2x88x92n)e+xe2x86x92SiF2+2H++nexe2x88x92
SiF2+2HFxe2x86x92SiF4+H2 
SiF4+2HFxe2x86x92H2SiF6 
or
Si+4HF+(4xe2x88x92xcex)e+xe2x86x92SiF4+4H+xcexexe2x88x92
SiF4+2HFxe2x86x92H2SiF6 
where e+ and exe2x88x92 represent a hole and an electron, respectively, and n and xcex are the number of holes necessary to dissolve one Si atom. According to them, when n greater than 2 or xcex greater than 4, porous Si is formed.
The above fact suggests that p-type Si having holes is converted into porous Si while n-type Si is not converted. The selectivity in this conversion has been reported by Nagano et al. and Imai (xe2x80x9cOxidized Porous Silicon and It""s Applicationxe2x80x9d K. Nagano et al., (The transactions of the institute of electronics and communication engineers), The Institute of Electronics, Information and Communication Engineers, Vol. 79, pp. 49-54, SSD79-9549, 1979), (xe2x80x9cA New Dielectric Isolation Method Using Porous Siliconxe2x80x9d K. IMAI, Solid-State Electronics, Vol. 24, pp. 159-164, 1981).
However, it has also been reported that n-type at a high concentration is converted into porous Si (xe2x80x9cComplete dielectric isolation by highly selective and self-stopping formation of oxidized porous siliconxe2x80x9d R. P. Holmstrom and J. Y. Chi, Applied Physics Letters, Vol. 42, pp. 386-388, 1983). Hence, it is important to select a substrate which can be converted into a porous Si substrate independently of p- or n-type.
To form a porous layer, instead of the above anodizing method, for example, a method of implanting ions into a silicon substrate may also be used.
For example, in the method described in Japanese Patent Laid-Open No. 5-021338, i.e., the method of preparing a substrate (to be referred to as a bonded substrate stack hereinafter) by bonding a first substrate having a non-porous layer such as a single-crystal Si layer on a porous layer to a second substrate via an insulating layer, and separating the bonded substrate stack at the porous layer so as to transfer the non-porous layer formed on the first substrate side to the second substrate, the technique of separating the bonded substrate stack at high reproducibility and high yield is very important.
The present invention has been made in consideration of the above situation, and has as its object to improve the reproducibility and yield in separating, e.g., a sample or composite member such as a bonded substrate stack.
According to the first aspect of the present invention, there is provided a separating apparatus for separating a sample by a fluid, characterized by comprising a holding portion for holding a sample having a separation layer inside, a nozzle for injecting a fluid to the separation layer of the sample held by the holding portion, a fluid supply portion for supplying the fluid to the nozzle, wherein the fluid supply portion suppresses a variation in pressure of the fluid to be supplied to the nozzle within a predetermined range during separation processing so that the fluid is ejected from said nozzle at substantially constant pressure.
The fluid supply portion preferably suppresses the variation in pressure of the fluid to be supplied to the nozzle within xc2x110% of a target pressure during the separation processing.
The fluid supply portion preferably comprises a servo-driven pump and supplies the fluid to the nozzle from the servo-driven pump.
The separating apparatus according to the first aspect of the present invention preferably further comprises a rotational drive portion for rotating the sample about an axis perpendicular to the separation layer by rotating the holding portion.
The separating apparatus according to the first aspect of the present invention preferably further comprises an operation mechanism for changing a position where the fluid is injected from the nozzle to the sample along with progress of the separation processing. The operation mechanism preferably changes the position where the fluid is injected to the separation layer of the sample gradually or stepwise from a peripheral portion to a central portion of the separation layer along with progress of the separation processing.
The sample preferably has, outside the separation layer, a concave portion recessed from a side surface.
The separation layer is preferably a fragile layer, e.g., a layer formed by anodization or ion implantation.
According to the second aspect of the present invention, there is provided a separating method of separating a sample by a fluid, characterized in that a sample having a separation layer inside is separated at the separation layer while injecting a fluid whose variation in pressure is suppressed within a predetermined range to the separation layer of the sample.
According to the third aspect of the present invention, there is provided a transfer method of transferring a transfer layer on a surface of a first member to a second member, characterized by comprising the preparation step of preparing a composite member by bringing the first member having a separation layer inside and the transfer layer on the separation layer into tight contact with the second member, and the step of separating the composite member at the separation layer while injecting a fluid, maintained at substantially constant pressure by suppressing variation in pressure within a predetermined range, to the separation layer of the composite member, thereby transferring the transfer layer of the first member to the second member.
According to the fourth aspect of the present invention, there is provided a substrate manufacturing method characterized by comprising the preparation step of preparing a bonded substrate stack by bonding a first substrate having a separation layer inside and a transfer layer on the separation layer to a second substrate, and the separation step of separating the bonded substrate stack at the separation layer while injecting a fluid, maintained at substantially constant pressure by suppressing variation in the pressure within a predetermined range, to the separation layer of the bonded substrate stack.
The predetermined range is preferably xc2x110% of a target pressure.
In the separation step, preferably, the pressure of the fluid is servo-controlled.
In the separation step, preferably, the bonded substrate stack is separated while being rotated about an axis perpendicular to the separation layer.
In the separation step, preferably, the bonded substrate stack is separated while changing a position where the fluid is injected to the bonded substrate stack along with progress of separation processing. In the separation step, more preferably, the bonded substrate stack is separated while changing the position where the fluid is injected to the separation layer of the bonded substrate stack gradually or stepwise from a peripheral portion to a central portion of the separation layer along with progress of the separation processing.
The separation layer is preferably a fragile layer, e.g., a layer formed by anodization or ion implantation.
The transfer layer preferably includes a single-crystal Si layer, and more preferably, in addition to the single-crystal Si layer, an insulating layer on the single-crystal Si layer.
According to the fifth aspect of the present invention, there is provided a semiconductor device manufacturing method characterized by comprising the steps of preparing an SOI substrate using the substrate manufacturing method according to the fourth aspect of the present invention, and element-isolating an SOI layer of the SOI substrate and forming a transistor on the element-isolated SOI layer.
The transistor may be either a partial depletion type FET or a complete depletion type FET.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.